1. Field of the Invention
This invention relates to a radiation image read-out apparatus for exposing a stimulable phosphor carrying a radiation image stored therein to stimulating rays which cause the stimulable phosphor to emit light in proportion to the radiation energy stored, and detecting and converting the emitted light into an electric image signal. This invention particularly relates to a radiation image read-out apparatus using a light source linearly emitting stimulating rays and a line sensor comprising many solid state photoelectric conversion devices as a photodetector for receiving the light emitted by the stimulable phosphor and converting it into an electric image signal.
2. Description of the Prior Art
A novel radiation image recording and reproducing system is disclosed, for example, in U.S. Pat. No. 3,859,527. The system comprises (i) exposing a stimulable phosphor sheet to a radiation passing through an object such as the human body to have a radiation image stored therein, (ii) scanning the stimulable phosphor sheet by stimulating rays which cause it to emit light in proportion to the radiation energy stored, (iii) detecting the emitted light and converting it into an electric image signal by use of a photodetector, and (iv) reproducing a visible image by use of the electric image signal.
In the aforesaid radiation image recording and reproducing system, during image read-out a semi-transparent mirror is positioned at an angle of 45.degree. with respect to the stimulable phosphor sheet, and stimulating rays are made to pass through the semi-transparent mirror from the back thereof and to impinge upon the stimulable phosphor sheet. Light emitted by the stimulable phosphor sheet in proportion to the stored radiation energy when it is exposed to stimulating rays is laterally reflected by the semi-transparent mirror and is received by an image intensifier tube or a photomultiplier. Alternatively, stimulating rays are emitted from the rear surface of the stimulable phosphor sheet via an aperture, and light emitted by the front surface of the stimulable phosphor sheet is laterally reflected by a prism and received by an image intensifier tube. However, since the semi-transparent mirror or the prism is positioned far away from to the stimulable phosphor sheet, it is not always possible to efficiently guide the light emitted by the stimulable phosphor sheet, which is non-directional and weak.
On the other hand, Japanese Unexamined Patent Publication No. 58(1983)-121874 discloses an X-ray image converter comprising a light sensor utilizing a photoconductive semiconductor instead of a photomultiplier or an image intensifier tube, the light sensor being overlaid on the whole surface of the stimulable phosphor sheet. The light sensor comprises two transparent electrodes, which may be divided into parallel strip-like portions, and the photoconductive semiconductor is sandwiched therebetween. Image read-out is conducted by scanning the stimulable phosphor sheet by stimulating rays made to impinge upon the sheet from the outside via the light sensor. Or, an array of light emitting diodes exhibiting a spectrum of stimulating rays are positioned over the whole surface of the light sensor, and the light emitting diodes are sequentially turned on to emit light for scanning the stimulable phosphor sheet. In the X-ray image converter, since the semiconductor layer is directly overlaid on the stimulable phosphor sheet, the possibility of a light receiving loss, i.e. of a loss of light emitted by the stimulable phosphor sheet in the space between the light receiving device and the stimulable phosphor sheet, is decreased. Therefore, it would be expected that the signal-to-noise ratio would increase.
However, actually, the aforesaid X-ray image converter has the drawbacks as described below.
(1) Since the light sensor is overlaid on the whole surface of the stimulable phosphor sheet, the photoconductive semiconductor is deteriorated when the stimulable phosphor sheet is subjected to noise erasing necessary for repeatedly using the stimulable phosphor sheet. (When the stimulable phosphor sheet is used repeatedly, radiation energy which remains on the stimulable phosphor sheet after the image read-out step and which constitutes noise in the next image recording and read-out is erased. Normally, noise erasing is conducted by exposing the stimulable phosphor sheet to a large amount of erasing light having a wavelength within the stimulating ray spectrum. Being exposed to the large amount of erasing light repeatedly, the semiconductor is deteriorated.) Also, since the weight and the volume of each stimulable phosphor sheet become large, the stimulable phosphor sheet becomes inconvenient for handling. Further, it is not always possible to actually overlay the light sensor on the whole surface of the stimulable phosphor sheet and to install the light emitting diode array over the whole surface thereof. Even when such a configuration can be realized, the cost of realizing the configuration is high.
(2) It is not always possible to obtain a photoconductive semiconductor exhibiting quick response characteristics. Therefore, it is not possible to increase the scanning speed of the stimulating rays (a light beam emitted by a stimulating ray source or light beams emitted by the light emitting diode array). Also, since the intensity of light emitted by a point light source such as the light emitting diode array is low, the exposure time at each portion of the stimulable phosphor sheet must be increased in order to cause it to emit light sufficiently. This also makes it impossible to increase the scanning speed.
(3) Even when the transparent electrodes are divided into parallel strip-like portions, the electrode area is still large. Therefore, a large dark current inevitably arises, and the capacitance is large. As a result, the signal-to-noise ratio cannot be improved so much.